1. Field of the Invention
The present invention relates to a film for shielding light (referred to as a “light shielding film” hereinafter) which is to be disposed inside a display device such as a liquid crystal display device, a plasma display device, an EL display device, or a CRT display device.
2. Description of the Related Art
A light shielding film for a display is a black frame disposed inside a display device such as a liquid crystal display device, a plasma display device, an EL display device or a CRT display device; a lattice-shaped or stripe-shaped black edge around pixels (the so-called black matrix) in such a display device; or a black dot pattern or black line pattern for TFT light shielding.
An example of the display device light shielding film is a black matrix formed around red, blue and green pixels of a color filter, which prevents a drop in contrast caused by light leakage.
Another example thereof is a light shielding film provided on a thin film transistor (TFT), which is used in an active matrix driving type liquid crystal display device. This light shielding film is provided so as to prevent degradation in image quality of the TFT associated with electric current leakage caused by light. These light shielding films usually have to have such a light shielding property that the optical density thereof is 2 or more. The color of the light shielding film is preferably black from the viewpoint of the display quality of the display devices.
Metal has been used for producing a light shielding film for a display device having a high light shielding capability. An example of the process for the production may comprise forming a metal thin film by vapor deposition or sputtering, applying a photoresist onto the metal thin film, exposing the photoresist layer with a photo-mask having a pattern suitable for a light shielding film for a display device, developing the photoresist, etching the naked metal thin film, and peeling the resist layer on the metal thin film to form a light-shielding film (see, for example, Color TFT Liquid Crystal Display Device, pp. 218-220 (Apr. 10, 1997) published by Kyoritsu Shuppan Co., Ltd), the disclosure of which is incorporated by reference herein.
Since the metal thin film is used in this process, a high light shielding effect can be obtained even if the thickness of the film is small. However, since the process requires a film-formation in vacuum such as the vapor deposition or sputtering and an etching, the cost is high. The process also has a problem that the resultant light shielding film has a high reflectivity since it is made of metal, and that only a low display contrast can be obtained under intense external light. For dealing with this problem, a method can be employed in which a low reflective chromium film (such as a bi-layered film composed of a metal chromium film and a chromium oxide film) is used. However, this method costs higher. Besides, Chromium, which is most frequently used in this method, has a drawback of imposing a large load onto the environment.
There is a technique of using carbon black to form a light shielding film for a display device having a low reflectivity (see Japanese Patent Application Laid-Open (JP-A) No. 62-9301, the disclosure of which is incorporated by reference herein. This light shielding film is a film obtained by applying a photosensitive resin composition containing carbon black onto a substrate, drying the composition, exposing the resultant film to light, and developing the film.
However, carbon black has a lower ratio of the optical density to the coating amount than in the case of metal particles; therefore, the thickness of the film made of carbon black is inevitably large when a high light shielding performance and optical density are secured. Furthermore, this technique has drawbacks that in the case that pixels in red, blue and green are formed after the light shielding film is formed, air bubbles develop and the pixels are likely to be irregular.